The present invention relates to high voltage power supplies used in X-ray imaging equipment, and more particularly to circuitry for controlling an inverter used in such power supplies.
X-ray systems, such as computed tomography (CT) imaging apparatus frequently incorporate means to sense the high voltage applied to the X-ray tube. An accurate indication of the high voltage is required to regulate the power supply and accurately control X-ray imaging quality. For that purpose, special high voltage resistors form a voltage divider in the power supply to provide a low voltage feedback signal which is proportional to the high voltage produced by the power supply. Unfortunately, the high voltage resistors are subject to long term degradation due to environmental factors such as high electrical field stress, carbon tracking and treeing, corona discharges and transient events, such as tube spits. This degradation manifests itself as a change in the resistance with time. In addition to manufacturing tolerances, these aging effects introduce gain errors into the high voltage portion of the feedback system, which require periodic adjustment of the feedback circuitry to maintain the X-ray imaging system in calibration.
To accommodate adjustment for these gain errors, a variable resistor previously had been incorporated in the low voltage portion of the feedback circuit to adjust for variations in the high voltage components. The adjustment of the variable resistor involved an iterative manual calibration procedure, which depending upon the experience and skill of the service technician, often was quite time consuming. In addition, the presence of the variable resistor in the circuit created another serviceability problem. Since the gain of the low voltage control system was intentionally altered to accommodate the gain errors in the high voltage sensor, the transfer function of the low voltage section was not precisely known. Therefore, failure in either the high voltage or the low voltage sections often was difficult to detect and isolate. In the case of a closed-loop control system, the lack of failure detection can have catastrophic results. At a minimum, difficulty in isolating the failure can cause field repair to be very time consuming and expensive.